1. Field of the Invention
This invention relates to a dry etching method and more particularly to a dry etching method in which after-corrosion in etching a layer of an aluminum based material may be prevented effectively and in which mask selectivity in case of using an etching mask formed of a silicon oxide (SiO.sub.x) based material may be improved.
2. Description of Related Art
As a metallization material for semiconductor devices, aluminum or an aluminum-based material, such as an Al-Si alloy with content of 1 to 2% of Si or an Al-Si-Cu based alloy further containing 0.5 to 1% of Cu, is extensively employed. However, as the junction becomes shallower and the contact hole size becomes finer in keeping up with the recent tendency towards higher integration of the semiconductor devices, there is an increasing risk of malfunction such as destruction or deterioration of the junction or increased contact resistance due to Al elution into a diffusion layer or Si segregation from the metallization material in the contact hole. For this reason, it has become customary to provide a barrier metal layer between the metallization material and a silicon substrate for preventing an alloying reaction therebetween or silicon segregation. This barrier metal layer is usually constituted by transition metals, transition metal compounds such as nitrides, carbides, oxynitrides or borides of transition metals, refractory metal silicides, or alloys thereof. The barrier metal may not only be in the form of a single layer, but may exist as a combination of layers of different kinds of materials.
Meanwhile, in processing the layer of the Al-based material, there is presented a problem of corrosion produced after the end of dry etching, that is after-corrosion, discussed in detail in, for example, pages 101 to 106 of "Semiconductor World", April issue, Published by Press Journal. The following is an outline of the after-corrosion.
Dry etching of the layer of the Al-based material is usually carried out using chlorine based gases, as exemplified by a gas mixture of BCl.sub.3 and Cl.sub.2, as disclosed in JP Patent Publication KOKAI No.59-22374 (1984). The result is that AlCl.sub.3 as a reaction product or decomposition products of the etching gases inevitably remain in the vicinity of the pattern after the end of etching. These products are not only adsorbed to the wafer surface but also occluded within the resist mask. If these chlorine based reaction products or etching gas decomposition products absorb the moisture in the air to form electrolytic liquid droplets, Al is eluted in these droplets to produce corrosion. Besides, while CCl.sub.x polymer formed by the reaction between the resist mask and the chlorine-based active species plays an important role as a sidewall protection film for assuring shape anisotropy, Cl derived from this polymer also becomes harmful residual chlorine after etching.
The problem of after-corrosion is felt more keenly since Cu started to be used as additive in the Al-based interconnection or metallization because CuCl as an etching reaction product is left in the pattern section due to its low vapor pressure and, if water is supplied thereto, a local battery is formed which has Cl.sup.- as an electrolyte and Al and Cu as electrodes.
If the above mentioned barrier metal structure, or a structure in which an amorphous silicon layer or the like is stacked as an anti reflection coating on the surface of the Al-based material layer for improving patterning accuracy, is used, the after-corrosion tends to be produced. Since the cross-section of the stacked structure of heterogeneous materials is exposed to the atmosphere as a result of patterning, Al elution is promoted due to local battery effects on formation of the above mentioned droplets. On the other hand, the micro gaps on the interfaces of heterogeneous metals provide sites for chlorine or chlorine compounds to be retained.
The after-corrosion is produced to a more or less extent in case of using bromine-based gases as etching gases by the mechanism described above. For this reason, chlorine and bromine are termed herein as halogens. However, fluorine is excluded from the generic term of halogen unless specified to the contrary.
As countermeasures for combatting the after-corrosion, there are known (a) a method of plasma cleaning using fluorocarbon based gases, such as CF.sub.4 or CHF.sub.3, (b) a method of ashing off the resist pattern by an oxygen plasma, referred to hereinafter as resist ashing, and (c) a method of plasma cleaning by NH.sub.3 gas followed by washing with water. These countermeasures are aimed at eliminating residual halogens. That is, the halogen compounds are converted into fluorine compounds for elimination thereof upon vaporization, the resist pattern itself containing a large quantity of the residual halogen is removed for eliminating a halogen source, the halogen compound is converted into inert compounds, such as ammonium halides or, concurrently with the above, AlF.sub.3 or Al.sub.2 O.sub.3 coatings are formed on the surface of the Al-based metallization layer for suppressing the after-corrosion.
However, the above mentioned countermeasures, aimed at eliminating the residuals halogen, are not fully effective to suppress the after-corrosion effectively.
There has also been made a proposal based on a concept different from the above concept of eliminating the residual halogen. According to this proposal, the wafer surface is coated with a carbonaceous polymer, after the end of patterning of the Al-based material layer, using a deposition gas such as CHF.sub.3. This technique enables moisture adsorption to be inhibited by the water-repellent carbonaceous polymer to protract the waiting time for the subsequent process step.
Although this method is highly effective, if executed appropriately, it is necessary to carry out resist ashing simultaneously if there is left a larger quantity of halogen. In this case, it becomes difficult to carry out the process due to contradictory requirements that higher temperatures are suited for ashing and lower temperatures are suited for polymer deposition.
As a further approach, it has also been proposed to use a mask of an inorganic material instead of the resist mask as described above. As to the mask of the inorganic material, the JP Patent Publication KOKAI No.60-33367 (1985) discloses a process employing an SiO.sub.2 mask. Although it is contemplated herein to achieve a high etching resistance, the process is thought to be essentially excellent as countermeasures against the after-corrosion because the SiO.sub.2 mask itself is incapable of occluding halogens, while a sidewall protection material such as CCl.sub.x polymer as a halogen source may not be produced.
However, in order that the process employing the SiO.sub.2 mask may be used practically, it is necessary to overcome the latent problem of the increased step level difference on the wafer surface.
It is virtually impossible to eliminate the SiO.sub.2 mask after etching of the Al-based layer because the underlying Al-based layer is usually an insulating film composed of SiO.sub.2 -based material and, if the SiO.sub.2 mask is removed, the insulating film is removed simultaneously. Thus the SiO.sub.2 mask is left and used as a portion of the interlayer insulating film covering the patterned Al -based layer. However, this tends to increase the step level differences on the wafer surface to render it difficult to achieve desired planarization by the interlayer insulating film.
Although this inconvenience may be obviated to some extent by diminishing the thickness of the SiO.sub.2 mask, it is difficult to diminish the SiO.sub.2 mask thickness under the current state of the art. A reducing gas, such as BCl.sub.3, is usually added to the etching gas for the Al-based layer for removing a native oxide film on a surface thereof. Since the SiO.sub.2 mask is partially reduced to Si by this reducing gas, the SiO.sub.2 mask is attacked by Cl* and thereby partially removed. The SiO.sub.2 mask needs to be thick enough to take account of the partial removal.